The invention relates to a receiver for recovering data from spread spectrum radio signals. The invention furthermore relates to a method for recovering data from spread spectrum radio signals.
Data is communicated using spread spectrum radio signals by combining the data with a spreading code which has an effect of increasing the bandwidth of the radio signals on to which the data and spreading code combination are modulated. Properties of the spreading code facilitate detection of the radio signals and recovery of the data at a receiver, despite the presence of contemporaneously detected like modulated spread spectrum radio signals generated by other transmitters. As a result, spread spectrum radio systems are used in mobile radio telephone systems to provide contemporaneous communication of data from a plurality of mobile stations. This is known as code division multiple access, and has been selected for both second and third generation mobile radio communication systems.
A characteristic of radio communications within a part of the radio frequency spectrum allocated to mobile radio communication systems, is that the radio signals propagate from transmitters to receivers via a plurality of paths. As a result, the radio signals detected by the receiver are superimposed. Where a temporal difference in the propagation time between at least two paths differs by more than a symbol period, inter-symbol interference results which must be mitigated in the receiver in order for data communicated by the radio signals to be recovered.
A known receiver which operates to recover data from received spread spectrum radio signals is known to those skilled in the art as a xe2x80x9crakexe2x80x9d receiver. The rake receiver is known to be provided with a plurality of rake fingers. Each of the rake fingers is assigned on a pre-determined basis to one of a plurality of delays corresponding to the relative propagation delay experienced by the radio signals travelling via propagation paths along which the received radio signals may have traveled between the transmitter and the receiver. Thus the rake fingers are positioned in a temporal relationship with respect to each other and within each rake a cross-correlator is provided to cross-correlate the received radio signals with a locally generated version of the user specific spreading code. The cross-correlation after a symbol period, generated by each of the rake fingers, is thereafter combined in order to generate an estimate of the data symbols communicated by the radio signals.
A disadvantage of known rake receivers is that substantial parts of the energy of the radio signals may reach the receiver via paths which have a propagation delay corresponding to a relative temporal displacement falling between the temporal position of the fingers of the rake receiver. As such self-interference occurs within the rake receiver in that the energy corresponding to paths not in correspondence with the temporal position of the rake fingers causes interference with the correlation of the received radio signals within the rake fingers. Additionally each path received by a finger even produces self-interference to all other fingers. The self-interference is governed approximately by the auto-correlation function of segments of the spreading code.
Another known disadvantage of the rake receiver is caused by what is known as the xe2x80x9cnear/farxe2x80x9d problem. The xe2x80x9cnear/farxe2x80x9d problem is known to those skilled in the art as an effect whereby radio signals transmitted by a transmitter close to the receiver, having a relatively strong received signal strength, have an effect of suppressing radio signals transmitted by a transmitter further away, having a relatively weak signal strength. This has an effect of further exacerbating self-interference, especially where several paths of approximately the same amplitude which are not in synchronization with the corresponding temporal position of the rake fingers are present in the received signals.
A spread spectrum radio signal is generated by combining the data symbols to be communicated with a spreading code and modulating the resulting combination onto a radio frequency carrier signal. The spreading code typically comprises a plurality of symbols known as chips which are combined with the data by modulating the spreading code with the data in some way. Furthermore, in order to provide appropriate spectral shaping, the chips of the spreading code are combined with a modulation filter such as, for example, a root raised cosine filter. A root raised cosine modulation filter is well known to those skilled in the art. It is known that by passing the received signal through a receiver filter having a corresponding root raised cosine filter, a raised cosine pulse shape results, with the effect that if the signal is sampled at the symbol rate or in this case the chip rate, then no inter-symbol interference is present in the received signal samples. This applies, of course, only under the condition that there is no inter-symbol interference caused by the channel.
Multi-path propagation is a characteristic of radio communications in a frequency band used by mobile radio communication systems. As a consequence, inter-symbol interference resulting from the transmit and receive filters will be present at each of the correlators of the rake fingers as a result of paths causing self-interference. In other words, the self-interference problem has the further effect that the received chips are superimposed, causing residual inter-symbol interference in the received signal. In order to prevent non-linear distortions by aliasing when decimating the signal to the chip rate a very high initial sample rate is required so as to allow a fine time resolution of the decimation process. In order to effect this time resolution, the received signal must be oversampled at the chip rate. However, since the chip rate is already many times greater than the symbol rate, oversampling at the chip rate is undesirable.
It is accordingly an object of the invention to provide a receiver for recovering data from spread spectrum radio signals and a method for recovering data from spread spectrum radio signals which overcome the above-mentioned disadvantages of the heretofore-known receivers and methods of this general type and which recover data from spread spectrum radio signals in which inter-symbol interference is substantially reduced, without requiring a sampling of the spread spectrum signal at a high rate.
With the foregoing and other objects in view there is provided, in accordance with the invention, a receiver for recovering data from spread spectrum signals, comprising:
an equalizer for filtering spread spectrum signals in accordance with an impulse response adapted to the spread spectrum signals and generating filtered signals, the equalizer mitigating at least a portion of an inter-symbol interference present in the spread spectrum signals; and
a data symbol estimator connected to the equalizer for estimating data by de-spreading the filtered signals with a spreading code.
In accordance with another feature of the invention, a receiver controller includes a data processor for adapting a plurality of impulse response coefficients for an impulse response estimate of a communications channel through which the spread spectrum signals have passed, the data symbol estimator detects data symbols from the spreading code in combination with the impulse response estimate.
In accordance with yet another feature of the invention, the data symbol estimator is a rake detector having at least one rake finger for correlating the filtered signals with respect to the spreading code at a delay determined from the impulse response estimate at relative temporal displacements corresponding to at least one propagation path and scaled by at least one of the impulse response coefficients.
In accordance with a further feature of the invention, the spread spectrum signals include a given signal formed with a pilot spreading code, the rake detector includes a pilot rake for identifying the at least one propagation path by comparing the spread spectrum signals to the pilot spreading code, and upon identifying the at least one propagation path, allocating the at least one rake finger to the at least one propagation path.
In accordance with yet a further feature of the invention, a decimator is connected between the equalizer and the rake detector for decimating the filtered signals and forming decimated signals having one sample per chip.
With the object of the invention in view there is also provided a method for recovering data from spread spectrum signals, which comprises:
filtering spread spectrum signals in accordance with an impulse response adapted to the spread spectrum signals for generating filtered signals by equalizing the spread spectrum signals with an equalizer for mitigating at least a portion of an inter-symbol interference present in the spread spectrum signals; and
detecting data from the filtered signals with a spreading code used for spreading a spectrum of the spread spectrum signals.
In accordance with another mode of the invention, the filtering step includes conditioning the spread spectrum signals such that a remaining part of the inter-symbol interference corresponds to at least one selected propagation path of the spread spectrum signals.
In accordance with another mode of the invention, the spread spectrum signals are sampled for generating sampled signals and the filtering step includes combining the sampled signals with a plurality of equalizer coefficients.
In accordance with another mode of the invention, an impulse response estimate of a channel through which the spread spectrum signals have passed is formed, and the filtered signals are de-spread with the spreading code and in accordance with components of the impulse response estimate, the spread spectrum signals have been generated with the spreading code.
In accordance with a further mode of the invention, the impulse response is scaled by at least one of a plurality of impulse response coefficients of the impulse response estimate, and the de-spreading step includes correlating the filtered signals in accordance with the spreading code at a delay determined from at least one component of the impulse response.
In accordance with yet a further mode of the invention, the impulse response coefficients are adapted with respect to time in accordance with a point in time at which data symbols are detected.
The invention is generally provided in a receiver having an adaptive pre-equalizer for pre-processing the received spread spectrum signal before it is further processed in a rake detector. The pre-equalizer reduces the amount of inter-symbol interference resulting from a number of strong propagation paths, to the effect that residual inter-symbol interference at the output of the pre-equalizer may be substantially mitigated by the rake detector, thereby facilitating detection and recovery of the data using the rake receiver. Any type of pre-equalizer such as a linear equalizer or a decision feedback equalizer may be used.
According to the invention there is provided a receiver for recovering data from received spread spectrum radio signals. The receiver includes a filter which operates to filter the received radio signals with respect to an impulse response adapted to the radio signals, and includes a symbol estimator coupled to the filter and configured to estimate the data by de-spreading the filtered radio signals with respect to a corresponding spreading code, wherein the filter (22, 24) is an equalizer (22), which operates to mitigate at least some inter-symbol interference present in the radio signals.
By providing a pre-equalizer before the rake detector, some and ideally all of the inter-symbol interference may be cancelled in the received spread spectrum radio signals, and any remaining inter-symbol interference cancelled by the rake detector.
The equalizer may operate to condition the remaining inter-symbol interference such that the remaining inter-symbol interference corresponds to paths at temporal positions corresponding to the temporal position of the rake fingers.
The equalizer may operate to convolve the received signals with a plurality of equalizer coefficients. The receiver may further include a data processor which operates to adapt the equalizer coefficients for minimizing an error signal derived from the received signals.
The receiver may be a linear equalizer or a decision feedback equalizer.
Linear equalizers and digital feedback equalizers are known to suffer error propagation and noise enhancement if configured to cancel all inter-symbol interference in a received signal. However the invention offers an advantage in that by only mitigating part of the inter-symbol interference in the received signal, the equalizer may operate according to a linear or a decision feedback equalizer whilst not incurring the disadvantage of noise enhancement or error propagation.
The equalizer offers a further advantage in that chip timing synchronization is substantially achieved by the equalizer which acts as an interpolation filter adjusting the delays for the subsequent rake detector. Furthermore, a synchronization may be achieved not only to the chip rate but also to the symbol rate, therefore obviating a requirement to acquire symbol synchronization after the received spread spectrum radio signals have been de-spread.
As will be appreciated, the invention can operate with an equalizer having fractionally spaced equalizer taps or T or chip spaced equalizer taps.
According to an aspect of the invention there is provided a method of recovering data from received spread spectrum radio signals. The method includes the steps of, filtering the spread spectrum radio signals with respect to an impulse response adapted to the spread spectrum radio signals, and detecting the data from the filtered signals in combination with a spreading code used to spread the spectrum of the radio signals, wherein the step of filtering the received radio signals, includes the step of equalizing the spread spectrum signals using an equalizer to the effect of mitigating at least some inter-symbol interference present in the signals.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a receiver and a method for recovering data from received spread spectrum radio signals, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.